It is known that during the casting of a strand, the strand produced in a steel strand casting installation must be braced or supported because of ferrostatic pressure from the liquid core in the upper part of the strand in order to avoid bulging and tearing of the partially solidified strand shell. Such bracing or support may be provided by means of rolls on an axle or shaft, or by means of plates. The latter have not been employed too successfully because they require channels for cooling water, which become clogged at times. Vapor formed in these clogged channels creates pressure which, in turn, may cause explosions.
If rolls are used to support the partially solidified strand, they may either be full width rolls extending transversely of the strand path and as wide as the strand itself or they may be divided into several rolls of narrower width. Full width rolls generally must be of diameters which increase as a function of increased distances from the casting level, due to the substantial bending stresses imposed by the strand. Thus, minimum axle distances between rolls, and thus unsupported lengths of the strand are largely predetermined by roll diameters. The advantages of full width rolls is that their bearings are laterally spaced at each side of the hot strand path, and are therefore not subjected to great thermal stress, but there are significant disadvantages resulting from the large sizes of these rolls.
If unsupported spans of the strand are to be kept as small as possible, it is necessary to use several smaller rolls dividing the support of the strand over the width thereof. Depending on the division of the rolls over the strand width, one or several step bearings for the shaft or axle are required. These bearings are very close to the hot strand, and must, therefore, be given special protection. In order to increase the distance from the strand and still maintain small unsupported spans, it has been suggested to use discs in the place of rolls, with the discs having larger diameters than the usual roll diameters. The discs of two longitudinally spaced adjacent shafts or axles would then intermesh (see Austrian Pat. No. 262,528; German Disclosure No. 1,952,633). This latter construction has the smallest practicable unsupported spans of hot strand, and consequently many points for support and, to a large extent, protection from strand breakage.
It has also been suggested to confine the entire ferrostatic pressure of the liquid core by means of a special design of the strand cross section itself, and to entirely eliminate any support. The strand is designed to be self-supporting by forming the sides of the strand in a concave manner. Theoretically, this results in vault-like side walls capable of containing the liquid core. By eliminating the need for external support, water cooling can be applied to the strand, facilitating a more intensive cooling which shortens the length of the liquid core. This, in turn, provides for a decrease in the dimensions of the casting equipment. Furthermore, the resistance or drag caused by the supporting rolls during extraction of the strand from the equipment is reduced, so that the weight of the strand itself is theoretically sufficient to transport the strand within the equipment. The relatively small lateral support pressures resulting from the concave design of the strand side walls are contained by lateral supporting rolls (German Paper No. 2,144,082). This construction has not proved entirely satisfactory, however, because the concave shape of the strand side walls alone does not adequately insure against the possibility of the liquid core breaking through the strand shell. It has been further suggested to provide, in addition to the concave shape, a lattice-like support of undefined length starting under the chill mold (German Disclosure No. 2,264,454 and 2,206,606). This support is constructed in a fashion that does not affect the intensive water cooling, so that the strand shell may quickly reach sufficient tensile strength for transport by the driving rolls in the processing line.
It has now been found, mathematically, that the concave shape of the strand side walls has little effect upon the stability of the strand cross section. This is due to the fact that, with high strand temperature and the resulting plasticity of the material, no notable supporting effect can be achieved. The only controlling factor is the thermal stress in the strand shell. This thermal stress depends on the cooling of the strand shell which, when splashing on cooling water, is limited by the familiar Leidenfrost phenomenon.